The induction of p53 and apoptosis in response to ionizing radiation and other stressors in vivo varies greatly between normal tissues, between cell types within a tissue, and between tumor types. Our long-term goal is to understand the basis of this tissue specificity. One approach is to analyze the tissue specificity of the p53 response pathway in genetic mutants of putative upstream regulators of p53, notably the P13K family members DNAPK and Atm. These analyses have shown that in response to gamma radiation (1) DNAPK is not required to upregulate p53 or apoptosis. In fact, mutation in DNAPK sensitizes cells, even p53 null cells, to apoptosis. This demonstrates a novel DNAPK dependent anti-apoptotic pathway. (2) Atm is required to upregulate p53 and apoptosis in some tissue, but is not required in all tissues such as epithelium indicating there are compensatory pathways to regulate p53 and apoptosis and the relative importance of these compensatory pathways varies between tissue types. (3) DNAPK and Atm functionally collaborate in that simultaneous mutation in both genes results in synthetic lethality early in embryogenesis. We propose to (1) determine if mutation in DNAPK can also radiosensitize p53 null tumor cells and to characterize this novel DNAPK dependent anti-apoptotic pathway, (2) determine if DNAPK, Atm and Atr are redundant in regulating p53 and apoptosis in vivo, (3) determine the morphologic and cellular basis of lethality of DNAPK Atm compound mutant embryos, and if altered regulation of p53 or apoptosis contributes to this defect. Understanding the functional interaction of the P13 Ks, in regulating p53, apoptosis, development and carcinogenesis at the level of the whole animal is a necessary link to apply knowledge gained from cell culture models to the clinic.